With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 comprises, in axial flow series, an air intake 1, a propulsive fan 2, an intermediate pressure compressor 3, a high pressure compressor 4, combustion equipment 5, a high pressure turbine 6, an intermediate pressure turbine 7, a low pressure turbine 8 and an exhaust nozzle 9.
Air entering the air intake 1 is accelerated by the fan 2 to produce two air flows, a first air flow into the intermediate pressure compressor 3 and a second air flow that passes over the outer surface of the engine casing 12 and which provides propulsive thrust. The intermediate pressure compressor 3 compresses the air flow directed into it before delivering the air to the high pressure compressor 4 where further compression takes place.
Compressed air exhausted from the high pressure compressor 4 is directed into the combustion equipment 5, where it is mixed with fuel that is injected from a fuel injector 14 and the mixture combusted. The resultant hot combustion products expand through and thereby drive the high 6, intermediate 7 and low pressure 8 turbines before being exhausted through the nozzle 9 to provide additional propulsive thrust. The high, intermediate and low pressure turbines respectively drive the high and intermediate pressure compressors and the fan by suitable interconnecting shafts.
The fan, compressor and turbine sections have alternating, axially spaced arrays of rotatable blades and static vanes. The rotatable blades are mounted to a disc, drum or ring either through attachment features, where a shaped element is secured in a complementary feature, or integrally, where the blade is formed with, or integrally joined to, the disc, drum or ring. For convenience in the rest of the specification the description and claims will refer to a disc but this term should be considered to extend to drums and rings. The term “integrally bladed rotor”, or IBR should also be considered to extend to integrally bladed drums (blums), integrally bladed discs (blisks) and integrally bladed rings (blings).
Integrally bladed rotors are rotors having integrally formed, or attached blades extending radially outwards from their circumference. The integrally bladed rotors are lighter than a similar component with detachable blades and offer superior airflow, tolerance and strength characteristics. Despite these advantages IBRs are not routinely used because damage to one blade could result in the entire rotor being scrapped.
Processes to rebuild or repair IBRs are now being developed but the processes leave a surface finish that is not optimum. It is therefore necessary to finish machine the surfaces to restore to an appropriate surface finish. Given that the aerofoils of the IBR are cantilevered any machining operation can be susceptible to vibration/chatter that also affects the final surface of the component. Due to the relatively narrow gap between blades and due to the need to have full 360 degree access to the blade conventional fixturing methods cannot be used to support the blade during the machining process.
It is an object of the present invention to seek to provide an improved method of supporting an aerofoil during machining.